A first known document-size detecting unit reads a position several millimeters or more than ten millimeters apart from an end of a platen glass (a platen) in a sub-scanning direction using a CCD image sensor and detects a document size.
However, in a case of the document not located in an area within several millimeters or more than ten millimeters from the end of the platen glass in the sub-scanning direction, the first known document-size detecting unit cannot determine the document size.
Thus, there is a need for an image reading apparatus that can determine the document size with a simpler structure even when the document is located apart from the end of the platen.